1. Field of the Invention
This invention relates to an optical element for correcting a chromatic aberration, an optical pick-up device having the optical element for correcting a chromatic aberration, an optical reproduction device having this optical pick-up device and an optical recording and reproduction device, and more particularly an optical element for correcting a chromatic aberration generated at each of optical planes in an optical system, an optical pick-up device having the optical element for correcting a chromatic aberration, an optical reproduction device having this optical pick-up device and an optical recording and reproduction device.
2. Description of the Related Art
ROM (Read-Only-Memory) type optical disc represented by a CD (Compact Disc), a RAM (Random Access Memory) type optical disc represented by a phase changing disc or a magneto-optic disc or an optical recording medium represented by an optical card and the like are widely used as storing media such as image information, audio information or programs for information equipment. In the case of these optical recording media, a high density and a large capacity have been gradually attained and in the case of the optical pick-up device corresponding to these optical pick-up devices, a short wave length formation of a light source, semiconductor laser, for example, or a large NA (Numerical Aperture) of an objective has been attained and a small diameter of a light collecting spot collected through the objective has been attained. For example, in the case of the CD which has been merchandised at a relative initial stage, a wave length of the light source is set to 780 nm, and to the contrary, in the case of DVD (Digital Video Disc or Digital Versatile Disc) which has been recently merchandised, a wave length of the light source is set to either 650 nm or 685 nm. However, in recent years, it has been desired to attain a higher density and a higher capacity of the optical recording medium and in correspondence with this trend, the wave length of the light source has been apt to show more and more a short wave length.
A chromatic aberration is an aberration which is generated when either a lens or an optical system must process either a multi wave length or a continuous wave length and a refractive index of optical material is made different in response to a wave length, resulting in that a focal distance of the objective is also made different. That is, since a refractive index of the optical material in a visual range shows a normal distribution, a refractive index for a blue light becomes a larger value than for a red light. For example, in the case of a convex type glass lens, a focal distance of blue light becomes shorter than a focal distance of red light. A wave length of laser light radiated from a semiconductor laser is generally a mono-chrome (a single mode) and it is assumed that there occurs no chromatic aberration, although actually it has a wave length width of about several nm or so. In addition, there is also a possibility of occurrence of a so-called mode hopping that a central wave length of a laser beam radiated from a semiconductor abruptly flies by several nm due to a variation in temperature or the like.
Accordingly, in the case that a short wave length semiconductor laser of about 440 nm or 440 nm or less, for example, is applied in an optical pick-up device corresponding to a higher density and a higher capacity of the optical recording medium, a chromatic aberration generated at the objective caused by a displacement of wave length becomes a non-allowable important problem. As to the fact that the chromatic aberration becomes high in response to a short wave length, it may be considered that there are two causes for it. A first cause consists in the fact that if a short wave length is applied in a usual objective, a variation of refraction index becomes large in respect to a minute variation in wave length and a defocusing amount which is an amount of motion of a focal point becomes large. A second cause consists in the fact that as a higher density and a higher capacity of the optical recording medium are attained, it is necessary to keep a diameter of a converging spot to be converged by the objective as less as possible, although as a depth of focus (d) of the objective is expressed by an equation of d=xcex/(NA)2 (where, xcex is a wave length xcex of a light source and NA is a numerical aperture of the objective), the depth of focus (d) becomes low as the wave length to be handled is short and even a slight defocusing is not allowed.
In order to keep the chromatic aberration of the objective low, it can be applied that optical material having a low dispersion property is used as the objective, although the chromatic aberration is even high under a short wave length. In addition, although it is possible that the objective is an achromatic lens composed of a plurality of lenses, a weight of the achromatic lens composed of a plurality of lenses becomes heavy. For example, in the case that the achromatic lens having a heavy weight is used as an objective and this objective is applied in a double-axis actuator for use in controlling and driving the objective in a focusing direction and a tracking direction, there occurs a possibility that servo quality such as focusing servo property and tracking servo property is reduced. In addition, in the case that the heavy achromatic lens is used as an objective, a high current is required in a drive power supply for the double-axis actuator and at the same time a drive circuit or a power supply or the like becomes large in size, resulting in that this may prohibit a small-sized formation of the optical pick-up device, the optical reproducing device provided with this optical pick-up device or the optical recording and reproducing device.
The present invention provides an optical element for correcting chromatic aberration in correspondence with a formation of short wavelength of the light source and provides an optical pick-up device, an optical reproducing device and an optical recording reproducing device in correspondence with a higher recording density and a higher capacity of the optical recording medium under application of this optical element for correcting chromatic aberration.
In order to solve the aforesaid subject matter, the optical element for correcting chromatic aberration of the present invention defined in claim 1 is arranged between a light source such as a semiconductor laser or the like with a wavelength being less than 440 nm and an objective with NA being 0.55 or more, with a focal distance being 1.8 mm or more and with an Abbe number at the d-line (Fraunhofer""s d-line of 587.6 nm, the color produced by an emission line of helium) being 95.0 or less, wherein the optical element for correcting chromatic aberration has a convex lens with at least an Abbe number of the d-line being 55 or more and a concave lens with an Abbe number of the d-line being 35 or less.
The optical element for correcting chromatic aberration of the present invention defined in claim 2 is arranged between a light source such as a semiconductor laser or the like with a wavelength being less than 440 nm and an objective composed of two lenses with NA being 0.70 or more, with a focal distance being 1.4 mm or more and with an Abbe number at the d-line being 95.0 or less, wherein the optical element for correcting chromatic aberration has a convex lens with at least an Abbe number of the d-line being 55 or more and a concave lens with an Abbe number of the d-line being 35 or less.
That is, the optical element for correcting chromatic aberration of the present invention defined in claims 1 and 2 is constructed such that the optical element is arranged between the light source and the objective irrespective of whether or not a flux of light at a position where the optical element is arranged is a converging flux of light or a diverging flux of light, a chromatic aberration of polarity opposite to that of the chromatic aberration generated at other optical surfaces is generated at an uneven surface through which the light passes. Accordingly, an uneven surface where the light passes through the objective and focuses at a focal point is kept at a state in which the chromatic aberration is cancelled, and an entire optical system becomes a system in which a superior correction of chromatic aberration is carried out within a range of variation of wavelength at the light source.
The chromatic aberration is generated when a refractive index (n) of optical material is changed only by xcex94n due to a variation in wavelength xcex94xcex of the light source. Then, a variation xcex94f of a focal distance (f) of a thin lens is given by a following equation (1):
(xcex94f/f)+xcex94n/(nxe2x88x921)=0xe2x80x83xe2x80x83(1)
wherein:
xcex94n/(nxe2x88x921)=(nFxe2x88x92nC/(ndxe2x88x921)=vd,
xcex94f=xcex94Fxe2x88x92C cause a following equation (2) to be attained,
vD is an Abbe number,
xcex94Fxe2x88x92C is a variation in focal distance at a line C and at a line F,
nF, nC, nd are refractive indexes against a Fraunhofer""s lines F (486.1 nm), line C (656.3 nm) and d-line (587.6 nm), respectively.
xcex94Fxe2x88x92C=xe2x88x92f/VDxe2x80x83xe2x80x83(2)
A chromatic aberration of the objective appears as an absolute value of variation in a focal distance (f) and as apparent from the above equation (2), the larger the focal distance (f), the larger a chromatic aberration, or the smaller the Abbe number VD of the optical material constituting the objective, the larger a chromatic aberration. Further, a depth of focus of the objective is given by d=xcex94/(NA)2, the shorter a wavelength or the larger a value of NA, the smaller a depth of focus, and the larger a deterioration of optical property caused by the chromatic aberration.
The optical element for correcting a chromatic aberration of the present invention has a configuration of two lenses in 1-group or one lens in 2-groups and it is preferable to make a configuration under a combination of a convex lens with an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less. A chromatic aberration can be expressed as a variation xcex94K of a refraction force K. When a refraction force of the objective is defined as K1, a variation xcex94K1 of the refraction force under variation of the wavelength is expressed by a following equation (3);
xcex94K1=K1xc3x97xcex94n/(n+1)xe2x80x83xe2x80x83(3)
If it is assumed that a variation xcex94n of a refractive index (n) of the optical material caused by a variation in wavelength of the light source is defined as xcex94n greater than 0, K1 is a positive value due to the fact that it is a refraction force of the converging lens and then xcex94K1 becomes xcex94K1 greater than 0 in reference to equation (3). In order to eliminate this state, it is satisfactory that an amount of variation xcex94K caused by a wavelength of a composite refraction force K=K2+K3 of the optical element for correcting a chromatic aberration comprised of two lenses in one group or two lenses in two groups expressed by a following equation (4) becomes xcex94K less than 0.
xcex94K=(K2/v2)+(K3/v3) less than 0xe2x80x83xe2x80x83(4)
wherein:
K2 and K3 are refraction forces of each of the two lenses in one group or one lens in two groups, and
v2 and v3 are Abbe numbers of each of two lenses in one group or one lens in two groups.
Further, in the case that the optical element for correcting a chromatic aberration is arranged in a beam with narrow widening angle radiated from the light source such as a semiconductor laser or the like or a substantial collimator optical path, the optical element for correcting a chromatic aberration has scarcely a power as compared with the refraction force K1 of the objective, resulting in that it is possible to assume that an equation of K=K2+K3=0 is attained and K2=xe2x88x92K3 is accomplished. Accordingly, equation (4) becomes as follows.
K2/v2 less than xe2x88x92K3/v3=K2/v3xe2x80x83xe2x80x83(5)
If it is assumed that K2 greater than 0 is attained, i.e. a convex lens is applied, a relation of K3 less than 0 is attained, i.e. a concave lens is applied and a relation of V2 greater than V3 is attained. To the contrary, if it is assumed that K2 less than 0 is attained, i.e. a concave lens is applied, a relation of K3 greater than 0 is attained, i.e. a convex lens is applied and a relation of V2 less than V3 is attained. That is, it is satisfactory that as a lens having a positive power, a crown glass, for example, is used, and as a lens having a negative power, a flint glass is used, and when a light source with a wavelength being 440 nm or less, it is necessary to make a quite high chromatic aberration to correct a chromatic aberration generated at each of the optical surfaces, so that it is desirable that a large difference is kept between the values of v2 and v3. For example, as a normal delustered tablet lens (a chromatic lens), a lens in which a convex lens of crown glass with a low dispersion and a concave lens of flint glass with a high dispersion are adhered to each other is applied. Although this lens by itself is a lens in which its chromatic aberration is restricted against a variation in wavelength, the optical element for correcting the chromatic aberration of the present invention by itself is required to generate mainly a chromatic aberration opposite to that of the objective so as to correct also the chromatic aberration of the objective and further it becomes necessary to provide a large difference in an Abbe number of the optical material constituting the lens. Then, since combination of the lens having an Abbe number of 55 or more and the lens having an Abbe number of 35 or less enables a chromatic aberration generated at the optical element for correcting a chromatic aberration of the present invention to be increased, a correction of the chromatic aberration generated mainly at the objective can be sufficiently carried out.
The optical pick-up device of the present invention defined in claim 3 is an optical pick-up device comprising a light source with at least a wavelength being 440 nm or less, an objective with NA being 0.55 or more, a focal distance being 1.8 mm or more and an Abbe number at the d-line being 95.0 or less, and an optical element for correcting a chromatic aberration arranged between the light source and the objective, wherein the optical element for correcting a chromatic aberration has a convex lens with at least an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less.
The optical pick-up device of the present invention defined in claim 4 is an optical pick-up device comprising a light source with at least a wavelength being 440 nm or less, an objective composed of two lenses with NA being 0.70 or more, a focal distance being 1.4 mm or more and an Abbe number at the d-line being 95.0 or less, and an optical element for correcting a chromatic aberration arranged between the light source and the objective, wherein the optical element for correcting a chromatic aberration has a convex lens with at least an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less.
The optical reproducing device of the present invention defined in claim 6 is an optical reproducing device comprising an optical pick-up device including a light source with at least a wavelength being 440 nm or less, an objective with NA being 0.55 or more, a focal distance being 1.8 mm or more and an Abbe number at the d-line being 95.0 or less and an optical element for correcting a chromatic aberration arranged between the light source and the objective, and a controlling and driving means for controlling and driving the optical pick-up device to a tracking direction, wherein the optical element for correcting a chromatic aberration has a convex lens with at least an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less.
The optical reproducing device of the present invention defined in claim 7 is an optical reproducing device comprising an optical pick-up device including a light source with at least a wavelength being 440 nm or less, an objective composed of two lenses with NA being 0.70 or more, a focal distance being 1.4 mm or more and an Abbe number at the d-line being 95.0 or less and an optical element for correcting a chromatic aberration arranged between the light source and the objective, and a controlling and driving means for controlling and driving the optical pick-up device to a tracking direction, wherein the optical element for correcting a chromatic aberration has a convex lens with at least an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less.
The optical recording and reproducing device of the present invention defined in claim 9 is an optical recording and reproducing device comprising an optical pick-up device including a light source with at least a wavelength being 440 nm or less, an objective with NA being 0.55 or more, a focal distance being 1.8 mm or more and an Abbe number at the d-line being 95.0 or less and an optical element for correcting a chromatic aberration arranged between the light source and the objective, and a controlling and driving means for controlling and driving the optical pick-up device to a tracking direction, wherein the optical element for correcting a chromatic aberration has a convex lens with at least an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less.
The optical recording and reproducing device of the present invention defined in claim 10 is an optical recording and reproducing device comprising an optical pick-up device including a light source with at least a wavelength being 440 nm or less, an objective composed of two lenses with NA being 0.70 or more, a focal distance being 1.4 mm or more and an Abbe number at the d-line being 95.0 or less and an optical element for correcting a chromatic aberration arranged between the light source and the objective, and a controlling and driving means for controlling and driving the optical pick-up device to a tracking direction, wherein the optical element for correcting a chromatic aberration has a convex lens with at least an Abbe number at the d-line being 55 or more and a concave lens with an Abbe number at the d-line being 35 or less.
Action caused by the aforesaid means will be described as follows. Although achromatic aberration generated at the objective becomes large as a wavelength of the light source becomes short, arrangement of the optical element for correcting a chromatic aberration of the present invention between the light source and the objective enables the chromatic aberration to be sufficiently corrected even if a central wavelength of the light source is 440 nm or less and has about several nm wavelength range, for example, or even if a central wavelength shows a mode hopping by about several nm due to a variation in temperature. In addition, it is possible for the optical element for correcting a chromatic aberration of the present invention to act as a function of a collimator lens constituting the optical pick-up device, i.e. a function for changing a radiated light from the light source into a parallel flux of light and then it becomes possible to perform a sufficient correction of the chromatic aberration without increasing the number of optical component parts constituting the optical pick-up device. Accordingly, in the case of the optical pick-up device provided with the optical element for correcting a chromatic aberration of the present invention, there is no possibility of occurrence of the chromatic aberration even if the light source of wavelength of about 440 nm or 440 nm or less is used and further it becomes possible that the optical reproducing device and the optical recording and reproducing device may accommodate for a higher recording density of the optical recording medium.